It is known that an open circuit voltage characteristic, which indicates a relationship between a state of charge and an open voltage, as one of characteristics of a secondary battery, is maintained as substantially the same open circuit voltage characteristic irrespective of deterioration of the secondary battery or changes in use condition (for example, refer to patent document 1). Using this characteristic, the patent document 1 discloses a battery capacity detecting method of assuming a state of charge at a time of measurement based on an open voltage measured during a charge or discharge stop period and an open circuit voltage characteristic. Further, patent document 1 discloses a method of estimating a full charge capacity of a secondary battery based on states of charge before charge start and after charge end and an amount of charged electric power supplied to the secondary battery during the charge, and also a method of estimating an amount of charge remaining in the second battery after charge is ended based on a state of charge after charge end and a full charge capacity.